1. Field of the Invention
The present invention relates to a method of regulating an alternating current rotating electrical machine, in particular a synchronous electric motor with smooth poles. It also relates to a device for supplying power to an alternating current rotating electrical machine.
It applies in particular, although not exclusively, to motors used in rail transportation for traction and active suspension.
To be more precise, the invention relates to a regulation method capable of slaving the electromagnetic torque of the machine to a set point torque.
2. Description of the Prior Art
The speed of a synchronous machine is generally controlled by regulating its electromagnetic torque. To this end, rotating machines are generally provided with a regulator receiving as input a set point electromagnetic torque and one or more sampled signals representing the electromagnetic torque of the machine and produced by measuring the stator current, the regulator applying a control voltage to a power supply inverter of the machine. To slave the electromagnetic torque of the machine to the set point torque, at each sampling time the regulator predicts the torque at the next sampling time and modifies the inverter control voltage accordingly.
A principal concern in most industrial applications is improving the dynamic performance of such machines and in particular increasing the torque dynamic range.
A first solution is to increase the switching capacity of the power switches of the inverter in order to increase the switching frequency. However, this solution implies using very costly power components and increases switching losses.
The system could use low-loss electronic components. This is unacceptable, however, because it considerably increases the cost of the power supply device of the machine.
There exist methods of defining the operation of a regulator using a continuous time model to model the operation of the machine, to carry out what is referred to as a xe2x80x9csynthesisxe2x80x9d by using the model to determine, again in continuous time, equations yielding the corrective control input to the system as a function of operating parameters required to obtain the specified operation, and then to convert the equations into a discrete time form to obtain differential equations that can be solved by a digital computer integrated into the regulator.
However, the performance of such methods is limited in terms of control dynamic range, and instability can occur if the sampling period imposed by the inverter is too long. Also, these methods cannot produce a set point torque within a single sampling period.
Patent application EP 1 045 514 discloses a method of controlling a rotating electrical machine using a discrete model of the machine and the power supply inverter of the machine, the model providing the control voltage to be applied to the machine by the inverter to achieve a set point torque and a set point magnetic flux modulus. However, the model is suitable for asynchronous rotating machines, and cannot be applied to any rotating machine regardless of its type. Furthermore, regulating the torque and the flux modulus does not take into consideration all the operating parameters of the machine. As a result of all this, the dynamic performance of the machine still has room for improvement.
An object of the invention is to overcome the drawbacks previously cited. That objective is achieved by providing a method of regulating a rotating electrical machine receiving as input a discrete control voltage determined to slave the electromagnetic torque delivered by the machine to a set point torque, said method consisting of determining at each sampling time kxe2x88x921 the discrete control voltage to be applied to the machine as a function of at least one sampled signal representing the electromagnetic torque of the machine, so that the set point torque is reached at the next sampling time k, which method includes:
a preparatory step of determining a discrete voltage control law of the machine, in which the discrete control voltage to be applied at each sampling time k is determined in the form of a first term corresponding to free evolution of the state of the machine in the absence of control, between the preceding sampling time kxe2x88x921 and the current sampling time, and a second term dependent on the set point torque and a set point for the magnetic energy consumed by the machine, and
at each sampling time, a step of determining, with the aid of the discrete control law, the control voltage to be applied to the machine for the torque of the machine to reach the set point torque and the magnetic energy consumed by the machine to correspond to the set point magnetic energy.
The set point energy is advantageously a minimum energy.
According to one feature of the invention the discrete control law determined during the preparatory step is of the form:
{right arrow over (V)}s,kxe2x88x921={right arrow over (f)}(xcex94xcex93k,xcex94Wk) 
in which f is a function giving the control voltage to be applied at sampling time kxe2x88x921 to reach the set point torque and energy as a function of variables xcex94xcex93k and xcex94Wk respectively representing the difference between the electromagnetic torque of the machine to be reached at the next sampling time k and the free evolution component of said torque at said time and the difference between the magnetic energy consumed by the machine at time k and the free evolution component of said energy at said time.
According to another feature of the invention the discrete control law is determined in a system of axes ({tilde over (d)}, {tilde over (q)}) fixed with respect to the free evolution in discrete time of the rotor flux of the machine.
If the machine is a synchronous machine with smooth poles the discrete control law determined during the preparatory step is of the following form:             [                                                  v                              d                ~                                                                                        v                              q                ~                                                        ]              k      -      1        =                    1                  a          ⁢                      "LeftDoubleBracketingBar"                                          Φ                →                            k              0                        "RightDoubleBracketingBar"                              ⁢              xe2x80x83            [                                                  Δ              ⁢                              xe2x80x83                            ⁢              W                                                                          Δ              ⁢                              xe2x80x83                            ⁢              Γ                                          ]        k  
in which V{tilde over (d)},kxe2x88x921 and V{tilde over (q)},kxe2x88x921 represent the components of the control voltage vector at time kxe2x88x921, expressed in a system of axes ({tilde over (d)}, {tilde over (q)}) that is mobile in discrete time and fixed with respect to the free evolution {right arrow over ("PHgr")}k0 of the rotor flux in the machine at the next sampling time k, ∥{right arrow over ("PHgr")}k0∥ corresponds to the modulus of the free evolution of the rotor flux at the next time sampling k, xcex94xcex93k and xcex94Wk respectively representing the difference between the electromagnetic torque of the machine to be reached at the next sampling time k and the component of free evolution of said torque at said time and the difference between the magnetic energy consumed by the machine at the sampling time k and the component of free evolution of said energy at said time.
The machine is advantageously a synchronous machine with surface-mounted permanent magnets.
Alternatively the machine is a synchronous machine with wound smooth poles.
According to one feature of the invention, when the machine is rotating at a speed less than a predefined threshold, the method includes implementing a low-speed strategy consisting of determining the control voltage to be applied to the machine to reach the set point torque at the next sampling time with zero magnetic energy input.
The low-speed strategy preferably consists of applying the following discrete control law:             [                                                  v                              d                ~                                                                                        v                              q                ~                                                        ]              k      -      1        =      [                                                      -                              I                                  k                  ,                                      d                    ~                                                  0                                      a                                                                          ΔΓ              k                                      a              ⁢                              "LeftDoubleBracketingBar"                                                      Φ                    →                                    k                  0                                "RightDoubleBracketingBar"                                                          ]  
in which Ik,{tilde over (d)}0 is the component of the free evolution of the stator current along the axis {tilde over (d)} of the system of axes fixed with respect to the free evolution of the rotor flux during the sampling period between the sampling times kxe2x88x921 and k.
According to another feature of the invention, when the machine is rotating at a speed greater than a predefined threshold, the method includes using a high-speed strategy consisting of taking account of limitations of the inverter to determine an intermediate set point torque that the machine can reach at the next sampling time with a given consumption of magnetic energy.
The high-speed strategy preferably includes solving the following system of equations:                               (                                    I                              k                ,                                  d                  ~                                                      -                          I                              k                ,                                  d                  ~                                            0                                )                2            +                        (                                    I                              k                ,                                  q                  ~                                                      -                          I                              k                ,                                  q                  ~                                            0                                )                2              =                  (                  a          ⁢                      "LeftDoubleBracketingBar"                                          V                →                                            s                ⁢                                  xe2x80x83                                ⁢                max                                      "RightDoubleBracketingBar"                          )            2                          I                  k          ,                      d            ~                          2            +              I                  k          ,                      q            ~                          2              =                  "LeftDoubleBracketingBar"                              I            →                                s            ⁢                          xe2x80x83                        ⁢            max                          "RightDoubleBracketingBar"            2      
in which Ik,{tilde over (k)} and Ik,{tilde over (q)} are the components of the stator current in the system of axes ({tilde over (d)},{tilde over (q)}) at the time k, Ik,{tilde over (d)}0 and Ik,{tilde over (q)}0 are the components of the free evolution of the stator current in said system of axes at the same time, ∥{right arrow over (V)}smax∥ and ∥{right arrow over (I)}smax∥ are respectively the moduli of the maximum voltage and the maximum current in the stator, and xe2x80x9caxe2x80x9d is a constant, the control voltage being obtained with the aid of the following equation             [                                                  v                              d                ~                                                                                        v                              q                ~                                                        ]              k      -      1        =            [                                                                                    I                                      k                    ,                                          d                      ~                                                                      -                                  I                                      k                    ,                                          d                      ~                                                        0                                            a                                                                                                            I                                      k                    ,                                          q                      ~                                                                      -                                  I                                      k                    ,                                          q                      ~                                                        0                                            a                                          ]        .  
The invention also provides a regulator for a rotating machine, including an inverter fed with a power supply voltage, a regulator receiving as input a set point electromagnetic torque and at least one sampled signal representing the electromagnetic torque of the machine and supplying to the inverter a control signal adapted to slave the electromagnetic torque of the machine to the set point torque by predicting, at each sampling time, the electromagnetic torque at the next sampling time and consecutively modifying the control voltage, which regulator includes:
a discrete control law for the machine, stored in memory, said control law giving the discrete control voltage to be applied to the machine at a current sampling time k in the form of a first term corresponding to the free evolution of the state of the machine in the absence of control between the preceding sampling time kxe2x88x921 and the current sampling time k and a second term dependent on the set point torque and a set point for the magnetic energy consumed by the machine, and
means for determining at each sampling time, with the aid of the discrete control law, the control voltage to be applied to the machine so that the electromagnetic torque of the machine reaches the set point torque and the magnetic energy consumed by the machine corresponds to the set point energy.
A preferred embodiment of the invention is described hereinafter by way of non-limiting example and with reference to the accompanying drawings.